Piperylene Photochemistry

The direct excitation of cis- or rron -piperylene (1,3-pentadienes) at 254 nm inefficiently results in cis-trans isomerization, hc- T = 0.09 and 4 t- c = 0.010 and very small amounts of 3-methylcyclobutene and 1,3-dimethyl-cyclopropene [(DMCP) = 0.002 4 o-.dmop = 0.009] 

These results were interpreted in terms of a cyclopropylmethylene mecham sm  

If the formation of (10) and (12) is more efficient than the formation of (11), then this mechanism accounts for the low quantum yield of stereoisomerization. 

While the direct cis-trans isomerization is inefficient, the sensitized reactions are very efficient Oc-t = 0.55 and 4 t-.c = 0.44, sensitizer is 

the photostationary state can be predicted from the ratio of the quantum yields  

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The photochemistry of o-hydroxy-aryl ketones and aldehydes has been extensively studied by several authors. It has been found that salicyl-aldehyde, 2-hydroxybenzophenone and 2,4-dihydroxybenzophenone are different from the unsubstituted compounds and from the p-hydroxy or o-methoxy derivatives in that they are completely inactive as sensitizers for the isomerization of piperylene [130]. Moreover, 2,4-dihydroxybenzophenone gives no luminescence in a rigid hydrocarbon glass at 77°K, whereas 4-hydroxybenzophenone exhibits an intense phosphorescence in... 

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